Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a copy of the desired image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. The various color toner can then be transferred onto a substrate in a superimposed registration so that a desired composite color image results. That composite color image can then be fused to make a permanent image.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is usually comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. The facets reflect the light onto the photoreceptor where a light spot is produced. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. Since the photoreceptor itself usually moves, the surface of the photoreceptor is raster scanned by the spot. During scanning the laser beam is modulated to produce a latent image on the photoreceptor. Then, depending upon the printer's configuration, the laser beam can be modulated to produce a latent image for each color of toner to be developed.
While raster output scanning is successful, it has problems. One set of problems results from facet imperfections. While each facet of the polygon is ideally both perfectly flat and perfectly parallel to the axis of rotation of the polygon, and while each facet is ideally identical to the others, and while each facet ideally forms the same angle with its adjacent facets, in practice these ideals are not achieved. Specifically important to the present invention are the facts that polygon facets are not perfectly flat and that the angles between adjacent facets are not identical. These imperfections cause the scan lines produced by different facets to be nonuniform in linearity and length and to start in different places. By using a start of scan detector the modulation of the various scan lines can be adjusted such that the latent image is aligned along one edge. That is, all scan lines start in the same place.
However, this does little to help other problems resulting from facet imperfections. For example, non-flat facets result in spatial nonuniformity: the spot is not where it should be at a particular time. Additionally, facet angle errors result in scan lines that trace across the photoreceptor at different rates and that end at different times. Color print testing performed at Xerox has proven that facet imperfections result in objectionable color defects that are referred to herein as color banding. Color banding results from color image to color image misregistration. Therefore, a technique of reducing or eliminating color banding would be beneficial.